I am a competitive junior player (17 y/o utr 11) and I am looking to develop a straight arm forehand to increase the power and spin on my forehand. Can anybody explain why the straight arm is better suited for more eastern grips or less severe semi western grips? (Delpo, Rublev, Federer, Janowicz, etc)
You seems to have some idea about it already coming in, the question you ask has a lot of assumption and prejudice.
Reading between the words, I guess maybe you have a disagreement with your coach, not sure about the details. Maybe someone told you something that straight arm is better for your game but you don't believe them. Or maybe you heard someone said eastern grip is better suited for straight arm but you are doing something that disapprove that statement and you just want some confirmation here in this forum.
I'm not sure, but I think that you are asking with an ulterior motive. I really don't care who you are proving this to but you should be asking for your own self not for the sake of arguments.
My answer to all your questions are follows:
1. straight arm or bent arm, it depends on how you hit, so whether it is eastern or more extreme it is not the right or relevant question to ask. I currently use 3/4 western grip for a relatively straight arm swing path. If you need an analogy with more famous pros, it might be closer to how Nadal/Murray do their forehand.
2. your goal is to be better at your competition, I guess you are not happy with your forehand, perhaps you want to hit winners but you generate too much spin so the ball doesn't travel fast enough. I would guess you think maybe straight arm might be better to increase that power and spin. I would say that that is also not the right question to ask, arm/shoulder are just one part of the swing, it is actually not the most decisive muscle groups that would give you what you want. I think you should think a bit more and ask better question.
Just my 2 cents. Hope this makes sense.